The Napoléon was a 90-gun ship of the line of the French Navy, and the very first purpose-built steam battleship in the world. She is also considered the first true steam battleship, and the first screw battleship ever. Launched in 1850, she was the lead ship of a class of 9 battleships, all considered as very successful and built over a period of 10 years. This class of ship was designed by the famous naval designer Dupuy de Lôme.

Before the experimental adoption of the screw in warships in the 1840s, the only available steam technology was that of the paddle wheels, which, due to their positioning on the side of the hull and the large machinery they required were not compatible with the broadside cannon layout of the battleships.
two views of the Napoléon. The rounded stern is visible.

"Dupuy de Lôme conceived and carried out the bolder scheme of designing a full-powered screw liner, and in 1847 Le Napoléon was ordered. Her success made the steam reconstruction of the fleets of the world a necessity. She was launched in 1850, tried in 1852, and attained a speed of nearly 14 knots (26 km/h). During the Crimean War her performances attracted great attention, and the type she represented was largely increased in numbers. She was about 240 ft (73 m). in length, 55 ft (17 m). in breadth, and of 5,000 tons displacement, with two gun decks. In her design boldness and prudence were well combined. The good qualities of the sailing line-of-battle ships which had been secured by the genius of Sané and his colleagues were maintained; while the new conditions involved in the introduction of steam power and large coal supply were thoroughly fulfilled."

Since 1843, large paddle-frigates were built. The famous Battle of Trafalgar proved that the paddle wheel, which had to be on each side of the hull, both reduced the number of guns that could be mounted broadside and was also a vulnerable target. Only with the invention of the screw propeller were steam-powered warships practical. Cannons would not be obstructed, and the underwater propeller was out of harm's way.

The screw propeller was first used to good effect on a military vessel in 1852 when Le Napoleon became the very first ship that was equipped with screw propeller technology. Although the British were better than the French in many nautical aspects, their screw -propeller battleships never surpassed Le Napoleon.

From 1844–45 the Anglo-French Entente collapsed following the French interventions in Tahiti and Morocco, and the publication of French pamphlets advocating a stronger navy (such as "Notes sur l’état des forces navales" by the Prince de Joinville), leading to an arms race in the naval area.
The United Kingdom already had a few coastal units with screw/steam propulsion in the 1840s, called "blockships", which were conversions of small traditional battleships into floating batteries with a jury rig, with a medium 450 hp (340 kW) engine for speeds of 5.8 knots (10.7 km/h) to 8.9 knots (16.5 km/h). However, the Napoléon was the first regular steam battleship to be launched.

In 1846, Britain had designed a screw/steam battleship named the James Watt, but the project was abandoned. Finally, the Agamemnon was ordered in 1849 and commissioned in 1853 as a response to rumours of the French development. Britain’s reluctance to commit to the steam battleship apparently stemmed from her commitment to long-distance, worldwide operation, for which, at that time, sail was still the most reliable mode of propulsion.

In the end, France and Great Britain were the only two countries to develop fleets of wooden steam battleships, although several other navies are known to have had at least one unit, built or converted with British technical support (Russia, Turkey, Sweden, Naples, Denmark and Austria). Altogether, France built 10 new wooden steam battleships and converted 28 from older battleship units, while Britain built 18 and converted 41.